The natural emergence or intentional release of a pandemic influenza virus has the potential to have catastrophic consequences for global public health. This is no better illustrated than by the 1918 Spanish influenza pandemic that swept the globe killing upwards of 50 million people. Although the mortality from subsequent pandemics has not been of this magnitude, other highly pathogenic influenza A viruses circulate in avian reservoirs. One such highly pathogenic virus, H5N1, is now entrenched in many regions of Asia and is growing in geographic range and numbers of hosts infected. Millions of poultry have been slaughtered in unsuccessful attempts to contain this virus. H5N1 infections in this outbreak have been confirmed in 116 humans of which 60 have been fatal. This situation has forced national and international agencies to develop strategic plans for a potential pandemic influenza outbreak. Most of these plans include the stockpiling of currently available anti-influenza drugs as well as the development of new ones. We propose to develop broadly cross-neutralizing monoclonal antibodies (MAbs) specific for H5N1 influenza viruses. This project brings together expertise in H5N1 virology, human MAb production, humanized MAb production and product development. Our approach will be to produce humanized and human (derived from vaccinees in the ongoing NIAID sponsored H5N1 vaccine clinical trials) MAbs capable of neutralizing most if not all Asian H5N1 strains. Antigenic analysis of murine MAbs and our experience with human MAbs demonstrate that broadly reactive MAbs can be made against drift variants of H5N1. This feasibility is shown with the recent isolation of a broadly neutralizing H5N1 MAb. A large panel of H5N1 MAbs will be generated, screened in vitro and in vivo for neutralizing activity, and then affinity matured. From these studies a lead product candidate will be selected for product development. The product will contain two MAbs to minimize escape mutants and to maximize neutralizing potency. We will also employ novel strategies for the production of second generation antibodies with unique properties. The lead antibodies and the second generation antibodies will be characterized in murine and ferret models of H5N1 infection to determine their therapeutic efficacy. The results of these studies will be the development of an effective, highly cross-reactive anti-influenza therapeutic for prevention and potentially for treatment of H5N1 infections.